1. Field of Invention
The invention relates to a highly flexible sheet composite material comprising a block flexible foam and a layer of whipped foam made of polyurethane-based dispersions two-dimensionally bonded thereto, a process for preparing such a composite material, and the use thereof.
2. Description of the Prior Art
Composite materials having a coating based on a whipped foam are widely known in the prior art. For the preparation of such composite materials, aqueous polyurethane or acrylate dispersions that first obtain a paste-like consistency by intensive stirring or whipping are employed. The process is very similar to the preparation of whipped cream. For this reason, the term “mechanical whipped foam” is also familiar. Usually, the foams are prepared with an air content of from 20 to 80 percent by volume. The density of the foams is stated as the foam weight per liter.
Although the aqueous dispersions alone easily form foams, additives are required for stabilizing the foams, such as ammonium stearate or SLES. According to need, the foams may also contain other aggregates usual in the coating field, such as color pigments, additives, active ingredients and fillers. The dispersions employed should have correspondingly high solids contents in order that a sufficient foam weight per liter can be achieved. In polyurethane dispersions, solids contents of from 40 to 60% are usual.
For producing the foams, continuously working industrial foaming aggregates as found in the food industry are employed. The application of the whipped foam is usually performed by a doctor knife, and the set coating gap essentially determines the wet film thickness.
The foams must be dried at higher temperatures to obtain ready-to-use coatings. To remove the water contained in the foam, the wet foam is dried in several steps at increasing temperatures in a forced air flow in such a way that the water will not boil and the foam structure is not destroyed. The end temperature in the drying process is around 160° C. The dried foams have closed-pore or open-pore structures depending on the foam weight per liter employed. The dry film thickness is usually slightly lower than the wet film thickness.
The layer thickness of the coatings is from 0.05 to 1.0 mm, but several millimeters is also possible. The foams are very much finer than the usual known foams as known from sponges and foam cushions.
The purpose of foam coating is to provide volume while the consumption of material and weight remain low. In addition, the coatings are very soft and have pleasant haptics due to the foam structure. Therefore, whipped foams are employed in clothing, in medical and technological articles, but also for the preparation of leather-like materials. In some cases, the foam must be provided with further solid layers for damage protection and for design reasons. Another advantage of the use of whipped foams is that they can be processed in an emission-free or at least very low-emission process.
Now, the coating of a material with such whipped foams to form a composite material can essentially be performed by two methods:                a) In direct coating, the whipped foam is directly bonded with the support material by suitable application methods. This is followed by drying at elevated temperatures in a range of from 140 to 170° C. over several minutes in order to solidify the whipped foam coating accordingly and to provide serviceability.                    In order to prevent the undesirable penetration of the applied whipped foam material into the substrate during the application and to be able to apply appropriate amounts, the viscosity and foam weight per liter is adjusted to obtain a paste-like consistency. In addition to the substrate, the type of doctor knife, which determines the coating height, and the dwelling time until the composite enters the drying line have an influence on the penetration depth.                        b) In reverse coating (also referred to as transfer coating or release (paper) coating), the whipped foam coating is first applied to a release paper, cured, and only in a second step is it bonded with the material to be coated. Subsequently, after drying and firmly bonding the coating to the coated material, the release paper can be peeled off and optionally reused several times. The advantage of this method resides, on the one hand, in the possibility of providing the coating with a defined surface structure, such as a grain structure, by pre-imprinting the release paper. On the other hand, however, the bonding to the support material can be controlled more easily as compared to direct coating. Therefore, the thus prepared articles are often substantially softer.        
However, such composite materials comprising a polyurethane-based whipped foam layer as known from the prior art have the disadvantage of having an insufficient flexibility for many applications. In particular, in many such composite materials, if used, for example, for cladding movable (machine) parts, it is a disturbing effect that creases are formed in the course of time, which promotes a faster wear of such composite materials.